Implementing a metal gate technology into metal-oxide-semiconductor field effect transistors (MOSFETs) not only improves a device performance, but also decreases the feature size. Generally speaking, the metal gate technology is a technology developed to replace the typically polysilicon gate electrode of a MOSFET with a metal gate (MG) electrode. One process of forming the MG electrode is termed “gate first” process while another MG electrode formation process is termed “gate last”. For the“gate first” process, the MG electrode of the MOSFET is deposited before the high temperature activation anneal of the semiconductor process while for the “gate last” process the MG electrode is deposited after the high temperature activation anneal. Although the thermal requirements are loosened in the “gate last” process, the MG electrode may be corroded by the acid solutions during the chemical mechanical polishing or planarization (CMP) process of the gate stack.
Like reference symbols in the various drawings indicate like elements.